Fiber laser devices can produce a small beam spot of light having high focusing performance and high power density, and process in a noncontact manner. Accordingly, fiber laser devices are used in various fields such as the laser processing field and the medical field. In particular, fiber laser devices used in the processing field and the medical field are high power fiber laser devices.
A fiber laser device includes an amplification optical fiber having a core coated with a clad and doped with an active element for amplifying light. Pumping light propagating through the clad of the amplification optical fiber is absorbed by the active element, whereby the active element is pumped. Light to be amplified propagating through the core is amplified by stimulated emission of the active element and output therefrom. It is therefore desirable that pumping light input to the amplification optical fiber be efficiently absorbed by the active element without any loss. However, it is known that skew mode propagation may occur in the amplification optical fiber, where part of pumping light propagates only through the clad and is not absorbed by the active element.
Patent Document 1 listed below discloses an amplification optical fiber in which such skew mode propagation is suppressed. In the amplification optical fiber, the cross-sectional shape of a clad is square and scattering means for scattering pumping light is provided at a portion of the clad. The amplification optical fiber thus scatters skew mode light so that the scattered light passes through a core.    [Patent Document 1] Japanese Patent No. 3479219